Anion-Induced Reversible Actuation of Squaramide-Crosslinked Polymer Gels

Supramolecular anion binding to squaramide crosslinkers in poly(N,N-dimethylacrylamide) gel networks enhances swelling and allows reversible chemically driven actuation. The volume swelling ratio of the gels is shown to depend on both the type of anion and its concentration. 1H NMR and UV–vis titrations with the squaramide crosslinkers reveal a relationship between anion binding affinity and the concentration-dependent swelling behavior. Gel swelling is shown to be reversible, and by embedding a solid support into rod-shaped gels, soft actuators are fabricated that undergo forward and backward bending motion in response to changing anion concentration. The swelling and bending process, which is accompanied by intense green coloration of the gel, is achieved by using only low amounts of crosslinker. This macroscopic actuation achieved by anion binding to specific molecular entities in the polymer network will open new opportunities in the field of chemically responsive materials.


UV-Vis Titrations
For UV-Vis titrations, 50 mM solutions of receptors (SQ1 and SQ2) and 5 mM solutions of the tetrabutylammonium anion were prepared in a mixture of DMSO and 0.5% H2O (v/v). Subsequently, the 5 mM anion solution was added stepwise to 2 mL of the 50 mM receptor solution and a UV/Vis spectrum was recorded after each addition. The data was fitted to a 1:1 binding model using the online software BindFit 2-3 with the absorption intensity being observed at a specific wavelength to track hostguest interactions accordingly.

Equilibrium Swelling
For the swelling experiments, gel specimens were first carefully removed from the glass vials (by breaking them). All gel specimens were weighed prior to the swelling experiment. The weight distribution, average and standard deviation of the cured gels were used as measure to assess successful polymerization of each batch and corresponding data is shown as candle stick diagrams

Mass & Volume Swelling Ratio
The mass and volume swelling ratios were calculated according to formulas used in the Flor-Rehner mesh size theory. [4][5] Accordingly, the mass swelling ratio Qm is defined as the ratio of ms, the gel mass in equilibrium swollen state, and the dry mass md.
(S1) Next, the mass swelling ratio Qm is used to calculate the volume swelling ratio QV (equation S2) where p and s are defined as the density of the solid polymer and the solvent (water), respectively.
The density of DMSO is s = 1.1 g/cm³ and the density of poly(N,N-dimethylacrylamide) was approximated to p = 1.213 g/cm³. 6

Rheology
For the rheological measurements, gels were prepared in glass vials as previously described. Three

Cycling Swelling Experiments
For the cycling swelling experiments, gel specimens were first carefully removed from the glass vials

Rod-like Gels using a PEEK Mold
A PEEK-based mold was created to prepare rod-shaped gel samples ( Figure S50)

S36
To assemble the PEEK mold, a small amount of silicon grease was applied to the flat surfaces around the chamber openings on both sides of the mold. Subsequently, glass objective slides were attached and a commercial black foldback clip was used to hold the setup together. After a rubber seal was attached on top and the inner volume was purged with argon for 2 mins, the degassed gel precursor solution was loaded into each of the chambers. The precursor-loaded PEEK mold was then placed in an oven overnight at 60 °C. For the gels used in the actuation experiments, at the flat bottom of each chamber, a thin filter paper strip (cellulose filter) was fixated prior to the loading of the chamber with the precursor solution. In this way, the filter paper was successfully embedded as a support layer into the gel.

Actuation Experiments
To induce actuation of the gel samples, they were furnished with a solid support. Synthesis of the rodshaped gels proceeded as previously described, except that at the lower bottom of each chamber a strip of filter paper was fixated before the PEEK mold was assembled, purged with Argon, and filled with the precursor solution. The cured gel was then carefully removed from the mold and the gel sample was cut into the desired dimensions (L⨯W⨯H) using a razor blade. Then the strip of filter paper ---now embedded into the gel ---was fixated with a tweezer. Next, the gel specimen was lowered to